Special applications include use in damp, or even wet applications, as are found in food plants, for example, where a salt spray might be used and produce a constant mist, or in chemical plants, or in manufacturing environments where volatile or inflammable solvents are used in the manufacturing process. In the damp or wet environments, the problem of corrosion exists with attendant reduction in the life of the fixture. In hazardous environments, safety requirements dictate that the possibility of an electrical discharge or spark be accounted for and either eliminated or encapsulated so that it is isolated from the environment in which the fixture is used. Alternatively, operating circuits may be designed to operate at inherently safe power levels, as discussed further below.
For brevity, reference will be made more frequently herein to hazardous environment application than wet, damp or other special applications. Persons skilled in the art will readily appreciate the facility with which the present invention is accommodated to many-different applications. One application where the present invention might have particular utility, as an example, might be a manufacturing plant for aircraft or a petroleum refinery where the use of volatile solvents and other flammable liquids or fumes are present. In most of these applications, it is desirable that the lighting be portable. From the user's standpoint, it is also desirable that the fixture be capable of being re-lamped without the use of special tools or devices because unless substitute lighting is available, when a lamp burns out, production may have to be curtailed or shut down, and safety may be compromised if supplemental light is not available.
Lighting has been designed for hazardous duty applications using incandescent lamps. However, incandescent lamps, particularly those capable of generating larger outputs of light, operate at fairly high temperatures, and therefore may create another potential hazard, particularly in an environment of volatile materials. Fluorescent lamps have also been incorporated in lighting for hazardous applications. However, fluorescent lamps typically require one hundred volts or more to initiate discharge, as well as for continual operation. Thus, precautions have to be made to reduce the possibility of arcing.
Conventional fluorescent lamps have electrodes passing through the glass envelope for connecting to the power source. In order to be able to replace the lamp, the electrodes are mounted in sockets in such a manner that they normally are exposed to the environment, again, unless special precautions are taken.
In some designs employing conventional fluorescent lamps, where leads, terminals, circuit elements or electrodes are exposed to the environment, designers have designed circuitry to operate at "intrinsically safe" power levels. This term is known in the art and refers to predetermined operating levels of voltage and current for switching circuits to insure that arcing will not occur. Although circuit designs can incorporate requirements for inherently safe circuit operation, that is not the case for fluorescent lamps and it becomes next to impossible to achieve an inherently safe control or ballast circuit for a conventional fluorescent lamp wherein the entire control and power system operates at inherently safe levels and still permit the fixture to be conveniently re-lamped. Thus, whereas operating or control circuits may operated at inherently safe levels, the power portions of circuitry for conventional fluorescent lighting cannot, and some other provisions (such as air purging) must be made for operation of conventional fluorescent lamps in hazardous environments.
One attempt to overcome the problems associated with operating conventional fluorescent lamps in a hazardous environment is described in the co-pending application of Baggio and Granat entitled AIR PURGED PORTABLE ELECTRIC LAMP, Ser. No. 431,308, filed Apr. 28, 1995. In that application, the fluorescent lamps and the power source are housed in an enclosure which is purged with breathable air before power is applied to the fluorescent lamp. Although these devices have been useful and represent an advance in the art, they require a separate source of breathable air, conduit or tubing routing the air from the source to the location of use, and circuitry for controlling the purging cycle and sensing when the breathable air is not flowing through the enclosure to purge the interior of the enclosure. Moreover, because the lamps are housed in a sealed environment except for the entrance and discharge of the breathable air, it normally requires that the fixtures be taken out of use and lamps replaced at a remote location where tools and the like are required.
Electrodeless lamp technology has been developed in which electrodes do not pass through the glass envelope of a fluorescent lamp. However, electrodeless lamp technology to date has been directed primarily to domestic or commercial applications in which the RF source, coupling mechanism and lamp are all integrated into a screw-type base so that it might replace the conventional incandescent lamp, such as is shown, for example, in U.S. Pat. Nos. 4,171,378 and 5,220,236. Other examples of the application of electrodeless lamp technology have characteristics similar to these two applications which prevent their use in hazardous or wet locations, for example, because the attempt has been to integrate the power source integrally with the lamp, leaving some portion of the input power supply lines, power supply or coupler in contact with, or not sealed from the environment in which the fixture is intended to operate.